With increasing natural gas demand in the United States, import of liquefied natural gas (LNG) has gained considerable attention. However, due to the variable chemical composition of LNG from various sources, the natural gas often requires additional processing steps to meet industry and environmental standards, especially in the US market. Therefore, most imported LNG is regasified for further processing (e.g., fractionation, or dilution with nitrogen), which requires substantial amounts of energy, but also provides significant refrigeration content. Numerous processes are known that couple regasification with other processes to reduce energy consumption and/or make use of the refrigeration content in LNG.
For example, regasification of LNG can be thermally coupled with a partial oxidation plant wherein at least a portion of the regasified LNG is employed for cooling and as a fuel in the burners as described by Child et al. in U.S. Pat. Nos. 5,295,350 and 5,394,686. While in such configurations the source of waste heat is relatively limited, regasification of LNG for production of natural gas in large quantities is typically not achieved. In other examples, as described in U.S. Pat. Nos. 4,036,028 and 4,231,226 to Mandrin and Griepentrog, respectively, published U.S. patent application 2003/0005698 to Keller, EP 0683847 to Johnson et al., and WO 02/097252 to Keller, heat for regasification of LNG is provided by a heat exchange fluid, which is in thermal exchange with a turbine exhaust or combined cycle power plant. While some of these configurations provide significant reduction in energy consumption for LNG reagasification, several difficulties nevertheless remain that have limited the full utilization of the LNG refrigerant content.
Most significantly, and among other difficulties, the heat transfer is at least in some of these configurations limited by the freezing point of the heat transfer medium. Furthermore, while the refrigeration content of the LNG is utilized to at least some degree, energy is not extracted from such configurations. Interestingly, European patent application EP 0496283 describes a system in which power is generated by a steam expansion turbine that is driven by a working fluid (water) that is heated by a gas turbine exhaust and cooled by a LNG regasification circuit. While such a configuration increases efficiency of a plant at least to some degree, several problems nevertheless remain. For example, the valuable cryogenic refrigeration content of the LNG is left unused as the freezing point of water (or water glycol mixtures) is relatively high.
Therefore, while numerous processes and configurations for LNG utilization and regasification are known in the art, all of almost all of them suffer from one or more disadvantage. Thus, there is still a need to provide improved configurations and methods for LNG utilization and regasification.